In a medium saucepan, heat soup and add cream cheese. Heat until cheese is dissolved into the soup. Dissolve gelatin into the soup mixture and add lemon juice, mayonnaise, onion, celery and shrimp. Cook until heated through.
Grease a 6 cup fish mold with extra mayonnaise or non-stick spray. Fill mold firmly with mixture. Smooth off the top and chill in refrigerator for 3 hours or overnight.
Invert mold onto a platter of lettuce and shake to release the mousse from mold. Garnish as desired.